A superconducting junction device is used in a condensed quantum state. Thus it is coupled to an adjacent external electromagnetic field coherently when junction voltage is applied, so it can be operated at high speed with less power. Therefore, a superconducting junction device can be used for various electronics fields, and it has been studied to be applied to a superconducting computer or a superconducting mixer. Especially after an oxide high temperature superconducting material with a critical temperature above liquid nitrogen was invented, the superconducting junction has been studied from the viewpoint of miniaturization of a cooling element or decrease of operating cost.
Oscillators using the superconducting phenomenon (hereinafter superconducting oscillators) are represented by a Josephson junction device comprising a laminated structure of a superconductor, an intermediate layer and another superconductor. The intermediate layer is composed of a material such as an insulator, a semiconductor, a normal conductor or a superconducting material. The current-voltage characteristic of the device varies depending on the kind of the intermediate materials. In a superconducting electrode of a Josephson junction device, the superconducting order parameter, whose amplitude is proportional to the square root of the number of superconducting particles, is substantially stabilized in its phase as well as its amplitude. On the other hand, the amplitude of the superconducting order parameter decreases in the intermediate layer. Therefore, when a voltage V is applied between the electrode (voltage stage), the voltage V is applied to the intermediate layer in which superconducting order parameters of the both superconducting electrodes are weakly linked. The superconducting particles injected from a superconducting electrode with higher energy to the other superconducting electrode through the intermediate layer release an amount of energy as a value of eV ("e" is an element charge of 1.6021892.times.10.sup.-19 C, and the energy eV is an outer voltage obtained when passing the intermediate layer). This phenomenon is due to the fact that the superconducting particles remain in the condensed state and in the lowest-energy state. The superconducting particles are in a strong coherent state and they demonstrate a wave motional characteristic. Therefore, the particles are transmitted to the other superconducting electrode due to the tunneling phenomenon, then directly couple to an external electromagnetic field without collisional relaxation with quasiparticles or phonons, and radiate photon energy. In other words, they radiate electromagnetic waves directly. When external electromagnetic waves irradiate to the Josephson junction device, the electromagnetic wave is partially resonant-absorbed at the tunneling intermediate layer. The absorbable wave is limited to have a frequency of f=2eV/h(Hz) of energy, equal to the energy difference eV, where h is Planck's constant of 6.626176.times.10.sup.-34 J.multidot.s. As a result, the current tunneling through the Josephson junction device increases by the rate of the absorbed photon numbers. This phenomenon is well known as the AC Josephson effect. Based on this principle, electromagnetic waves with precisely-defined oscillation frequency are irradiated, and a terminal voltage of a Josephson junction device which is constant current-biased is principally determined to be used as a voltage standard element. On the other hand, the Josephson junction device is applied with a constant voltage bias and used as an oscillator. In general, the impedance of a Josephson junction is low. Thus for a practical use, it is necessary to compose serial-parallel arrays by connecting many Josephson junction devices with similar characteristics in order to coordinate with load impedance, to supply power to the oscillation output, and to increase the standard voltage level of the voltage standard element.
FIG. 21 is a partially cutaway view in perspective of a Josephson junction array which is used for a traveling wave type voltage standard element ("Superconductor Electronics: Fundamentals and Microwave Applications" by J. H. Hinken, translated by Takuo Sugano, published by SPRINGER VERLAG Tokyo in 1992, FIG. 4-2 p.85) As shown in FIG. 21, a ground plane 112 is formed on a substrate 111, whereon a Josephson junction array is formed as a part of the transmission line. The Josephson junction array comprises dielectric 113 (specific dielectric constant .epsilon..sub.r1 =5.7, thickness d.sub.1 =1 .mu.m), a lower electrode 114, a tunnel oxide film 115 of a lead oxide film and an upper electrode 116. The dimension of each Josephson junction is predetermined to be 27 .mu.m (length).times.70 .mu.m (width), and the repeat length l.sub.k is determined at every 100 .mu.m, in order to prevent out-of-phase coupling between the Josephson junctions. Such an element is constructed to avoid out-of-phase coupling between the Josephson junctions by standardizing the characteristics, so that the operation can be stabilized.
An oscillator is also applied with Josephson junction arrays. For such an oscillator, the characteristic is determined by synchronizing the phases of the electric waves emitted by each of the Josephson junctions connected in series or in parallel (cf. "Superconducting Device", edited by S. T. Ruggiero and D. A. Rudman, Academic Press, Inc., 1990 p.146-165). In this case, the characteristics of the elements should be similar to stabilize the operation, so that the phase difference between the Josephson oscillating currents of fundamental mode flowing in each of the junction elements and the induced current with the external electromagnetic field will be fixed. As a result, a phase synchronization is realized between the currents flowing in the Josephson junction elements. In other words, the Josephson junctions are weakly linked to each other via the currents flowing in the junctions or via oscillating electromagnetic field of the outer parts. When the element is used, the oscillation voltage and the current of the junctions are phase-synchronized to each other. According to this embodiment, the oscillation power is added by the number of the Josephson junctions, so a higher power level is realized. Moreover, the impedance between the array terminals can be greater, so the matching to the transmission line impedance is improved.
In the above examples, the characteristic of a single Josephson junction is effectively used, while the non-linear characteristic is not changed.
Another superconducting element is explained below. A non-linear element of FIG. 22 is indicated in Published Unexamined (Kokai) Japanese Patent Application No. Hei 6-5937. In FIG. 22, a first weak link-type Josephson junction and a second weak link-type Josephson junction are disposed on a substrate 206, and the junctions are connected by an intermediate bridge 203. The first Josephson junction comprises a first superconducting electrode 201, a first weak link 204 and the intermediate bridge 203 (superconductor). The second Josephson junction comprises a second superconducting electrode 202, a second weak link 205 and the intermediate bridge 203. If the superconducting order parameter in the intermediate bridge 203 changes periodically in the space, the critical current density in the intermediate bridge 203 has a value larger than the value determined by its own material constant. As a result, the voltage between the superconducting electrodes 201 and 202 decreases. In other words, a negative resistance area appears in the voltage-current characteristic. The material of the substrate 206 is strontium titanate. The superconducting thin films of the superconducting electrodes 201 and 202 are YBCO-based oxide superconducting thin films having a thickness of 50-150 nm and a critical temperature of 90 Kelvin. The weak links (204, 205) are YBCO-based oxide superconducting thin films having a thickness of 0.5-2 nm and a critical temperature of 70 Kelvin. The intermediate bridge 203 is a YBCO-based oxide superconducting thin film having a thickness of 5-100 nm and a critical temperature is 80 Kelvin. The interval between the first and second superconducting electrodes is 5 .mu.m. Since the intermediate bridge 203 is a superconductor, shielding effect will occur due to the Meissner effect. Thus magnetic flux coupling or electromagnetic coupling is negligible between the first and second weak links (204, 205).
A very thin (1-2 nm) and uniform insulator tunnel barrier should be formed to obtain the switching characteristic using a single Josephson junction device. However, it is very difficult and the manufacturing process yield is low. For the present, only niobium superconducting material is suitable for this purpose. It is difficult to form a tunnel barrier of an insulator, using other superconducting materials like oxide superconductors such as YBCO base, Bi oxide (BSCCO) base or thallium base material. When a single Josephson junction is used as an oscillator due to its AC Josephson effects, the oscillating fundamental frequency is mainly determined to be 483597.9.times.V.sub.appl (GHz) where the applied voltage of the Josephson junction is V.sub.appl (V). This is the reason why the Josephson junction is used as a voltage standard element. However, the oscillating frequency will change according to the voltage deviation. To solve this problem, the element needs a precision standard voltage source to stabilize the oscillation frequency, but it is not preferable for a practical use. The same problem is found in a Josephson junction array having the same critical current and normal resistance.
The non-linear characteristic element is disclosed in Published Unexamined (Kokai) Japanese Patent Application No. Hei 6-5937. The characteristic depends on the junction interface between the superconductor and the weak links. Though the inventors manufactured the disclosed element using advanced junction-forming skill, the characteristic disclosed in the reference was not obtained. When the cross-section of the element was examined through a microscope, interfacial irregularity in the atomic order was found. This might be the reason why the negative resistance as is mentioned by the inventors was not obtained. According to Ginzburg-Landau theory relating to this reference, the connecting condition of the order parameter between the superconductor and the weak links at the interface determines the shape of the superconducting order parameter in the superconductor (including the intermediate layer). If the interface is not well-formed, the order parameter at the interface decreases. Thus a monotonous damping solution (or monotonous increasing solution) in a lower energy level is realized as a stable state, compared to another solution which periodically changes in a space of higher energy level. As a result, the non-linear characteristic of this reference is not observed when such an interfacial deterioration exists. In such a case, the characteristics of the two independent Josephson junctions are added up in volume. According to the reference, the junction interface accuracy should be strictly controlled, and the manufacturing is extremely difficult.
The AC Josephson effect of a single Josephson junction is used when an oscillator using a superconducting element is prepared. In such a case, the instantaneous oscillation frequency changes in proportion to the voltage. Thus the voltage variation and the voltage noise cause the variation of oscillation frequency and the change of oscillation output line. Therefore, an ultra-low impedance drive using a precision standard voltage source is needed to construct an oscillator with stable frequency, using a conventional Josephson junction or Josephson junction array. The practice is difficult or costly. Furthermore, the output oscillation is emitted by radiating to free space, so the method is inconvenient. In order to compose an oscillator which enables driving a load circuit, the circuit is constructed so that the bias voltage is applied through a load impedance etc. However, other problems are found in the method. For instance, the Josephson oscillation becomes a non-linear oscillation at an instantaneous frequency in proportion to the instantaneous voltage applied to the junction terminal by a load impedance, whereby the frequency spectral purity of the oscillation is decreased. As a result, the oscillation output power is lowered if it is passed through a band-pass filter with high Q value to raise the frequency spectral purity.
Furthermore, the amplitude of the oscillating current coupled with a tank circuit with high Q value is permitted to change only in a narrow range because of the non-linear oscillation. The range depends on the floating impedance of the bias circuit and the junction, thus the oscillation output power is as small as 10.sup.-7 W or less (cf. "PHYSICS AND APPLICATIONS OF THE JOSEPHSON EFFECT" by A. Barone and G. Paterno, translated by Takuo Sugano, Hiroshi Ohta and Tsutomu Yamashita, published by Kindaikagakusha Shuppan in 1988, p.289). Moreover, the impedance of the Josephson junction is as small as several .OMEGA. or less, so the impedance cannot be matched with that of free space one or with that of transmission circuit one. In a case of space radiation, the output level which is obtainable is only 10.sup.-3 to 10.sup.-4 times the oscillation amplitude at most.
When the Josephson junctions are formed in an array, problems are found with respect to the frequency variation as in the case of a single Josephson junction, which is obvious from the operation principle. It is necessary to manufacture dozens or hundreds of Josephson junction devices having the same characteristics and connect them in order to match the impedance for the purpose of improving the output coupling. As a result, the superconducting oscillator becomes bigger (with larger area) and the manufacturing yield deteriorates.
In addition, an element must have the negative resistance characteristic at the operation frequency if an oscillator is composed by using the conventional element having the negative resistance characteristic. Texts of electric circuit generally teach that this condition is an absolute requirement.